Prepreg materials have been used for many years to initially bond inner-cooling channels to high voltage coils prior to the application of a groundwall insulation. Generally, a fibrous mat made of glycol ethylene terephthalate polymeric ester fibers, such as disclosed in U.S. Pat. No. 2,465,319, is treated with a polyester resin. The polyester treated fibrous material is then interpositioned between the inner-cooling channels and coil strips during their assemblage. The assembly is heated to cure the prepreg and sufficiently bond the coils and channels together to permit handling of the assembly during application of an outer insulation. The assembly is then provided with a groundwall insulating cover usually comprising a number of wrapped layers of mica tape.
Because the mica insulation is relatively brittle, the entire assembly is impregnated under pressure with a polyester impregnant which, when cured, provides the required strength and stability as well as a bond between the groundwall and coil strips. Recently, however, a new resinous impregnant was developed in order to improve the hydrolytic stability of the high voltage coils. The new styrene-epoxy-anhydride impregnant, as for example set forth in U.S. Pat. No. 3,657,196, was found to provide not only the desired hydrolytic stability, but also to impart the desired tensile strength to the mica groundwall covering. Moreover, the styrene-epoxy-anhydride impregnant was found to have better solvent resistance than the polyester impregnant and is, therefore, replacing the polyester resins as an impregnant for high voltage devices. During the manufacture of the devices, however, it was discovered that the polyester treated prepreg was incompatible with the new resin resulting in delaminations between the groundwall and the inner coil assembly.
Accordingly, it is an object of the present invention to provide a resin treated fibrous mat prepreg which is not only compatible with styrene-epoxy-anhydride impregnants, but which has a shelf life of at least six months. A further object of the invention is to provide a prepreg which is nontacky and flexible and when cured possesses both good electrical and mechanical properties.